On 1/17/2012 11:06 AM, Jim Brown wrote:
> On 1/17/2012 3:40 AM, Paul Christensen wrote:
>> That ties in with why we have a 50-ohm coaxial line standard for most 2-way
>> communications purposes and it probably dates back to the Quackenbush era.
> I don't think so. 50 ohms was the standard for VHF/UHF communications
> gear because it more closely matched the impedance of the most common
> antennas -- a quarter wave vertical with radials, or a quarter wave on
> the roof of a vehicle. It did NOT become common in ham radio until the
> days of solid state rigs with fixed tuned output stages, and the
> designers of those rigs chose 50 ohms.
Log before that... EF Johnson, Collins, Hallicrafters, Globe,
When I was first licensed in 61 only a couple of hams in the county (and
adjacent county) used open wire feeds. The rest used coax so at least in
our area of the state coax was already the prevalent feedline.
> When I started in ham radio as a kid in the 50s, we used mostly RG59,
I don't know what the most used coax was back then as nearly all of us
used surplus via the MARS program and it had no markings on it. I do
know that it was 50 ohms and the size of RG-8. There were a lot of home
brew amps too with 304TLs and 813's being popular although it seems like
a few were using 4-400s with a couple of the well heeled using 4-1000s.
> because it was closer to the feed impedance of a half wave dipole in
> free space. Over the years, ham literature like ON4UN's book, which
> plots the feedpoint impedance of a half wave dipole at various heights,
> has made us more aware that 50 ohms cable is a better match to a low
> half wave dipole. I feed my 110 ft high dipoles with 75 ohm coax, and my
My slopers are nearly a perfect match for 50 ohm at resonance. The one
for 40 is about a 60 degree angle with the feedpoint a good 70 feet up.
I expected that for the 75 but not 40.
> low ones (on Field Day, for example) with 50 ohm coax.
> Another point. The BEST RG11-size cables have lower loss than the BEST
> RG8/RG213-size cables by virtue of their impedance, but RG-11 cables
> with smaller center conductors or with copper coated steel center
> conductors have significantly more loss on the lower HF bands than those
> with a big (#14) solid copper center.
I used a lot of 3/4" CATV hard line over the years (Connectors are easy
to make too). I had nearly a 1000 foot spool. I can't remember what I
swapped for it, but back when they were first installing cable they had
a spool fall off one of the trucks. It put a big dent in the outside
layer, but that was the only damage so there was over 900 feet of brand
new "good stuff"<:-)). However they were installing 1000' lengths and
gave it to my friend who asked what they were going to do with it. I
never had much luck with the smaller 1/2" stuff as I found it to be too
fragile. I came to the same conclusion with 1/2" Heliax. Good stuff,
but I would end up kinking it one way or another.
> There's a tutorial on coax and stubs on my website.
> 73, Jim K9YC
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